Minor cleanup from if to guard statement

I think the guard is a lot easier to read here, noticed while reviewing #3171.

Author: ahoppen

Sources/SwiftParser/Lookahead.swift

@@ -266,7 +266,7 @@ extension Parser.Lookahead {
     }
 
     // If we don't have attributes, then it cannot be an accessor block.
-    if nextToken.rawTokenKind != .atSign {
+    guard self.at(.atSign) else {
       return false
     }